Most tablet coating is still done using the same method as in the last 50 years, i.e. coating in the pan coater, in spite of the fact that this apparatus has serious drawbacks.
These drawbacks are due to the fact that in the pan only one side of the tablets' surfaces is exposed to a spray of coating liquid at a time. This apparatus also has the drawback that the inlet temperature of the drying gas has to be lower than the maximally permitted product temperature. This makes the evaporation capacity of the process gas low necessitating a low spray rate and resulting in a long process time. Furthermore, it is necessary to apply a very moderate spray rate to prevent the tablets from sticking together by the coating which fact also decreases the handling capacity.
Because of these drawbacks associated with the pan coater several processes have been suggested for coating particulate materials or small bodies, such as granulae, pellets or crystals.
The first improvement was the use of a fluidized bed for suspending the product. The coating solution was applied to the product as spraying from the top counter-current to the air flow. In comparison to the pan coater, the drying capacity was increased due to the drying capability of the fluidizing air. However, the inlet temperature of the drying/fluidizing air was limited by the maximally acceptable product temperature.
To improve the efficiency of coating it is suggested in U.S. Pat. No. 2,648,609 (Wurster) to impart a turbulent flow of the drying and suspending air by conducting it through ducts in a rotating disc before introduction below a screen over which pass the tablets being coated. The purpose of using a turbulent air flow was to obtain a tumbling action on the tablets to make the coating thereon more even. By this process the coating liquid was applied cocurrently to the air flow, enabling higher inlet temperatures of the drying air, but the treatment was rather severe to the tablets due to contact between the tablets during their tumbling movement. Besides, said tumbling created by the turbulent flow of drying air was insufficient to ensure an even distribution of the coating spray on all surfaces of each particle.
Moreover, processes involving a proper fluidisation of the articles to be coated are less suitable for tablets of the size usual inter alia in the pharmaceutical industry because, given their size and shape, these cannot easily be fluidized. Therefore, the fluidized bed was modified into a so-called spouting bed. In this design, the perforations in the bottom of the bed for the process air are concentrated in one or more locations so that the process air at those points has enough velocity to transport the tablets pneumatically. The spray nozzle is placed in the bottom of the fluid bed at the same place as the perforations. The coating solution is then applied in the same direction as the movement of the tablets, i.e. co-currently. With the process air entering where the spray nozzle(s) are placed and thus having the product, spray droplets and drying air all moving in the same direction, the heat and mass transfer are efficient. This change in design also permitted the inlet temperature to be higher than the maximum acceptable product temperature because the evaporation heat cooled the product. Although this design was more efficient than the previous designs, it had a rather limited equipment capacity. The product layer thickness was limited because the process air had to keep the tablets spouting. Also there had to be a minimal distance between the nozzles to avoid interference. An apparatus of this design is described in U.S. Pat. No. 4,749,595 (Honda et al.).
Also U.S. Pat. No. 5,145,650 (Huttlin) discloses a fluidized bed apparatus having a plurality of nozzles. Although the area of applicability is indicated as including tablet coating, the apparatus seems most suitable for processing and agglomerating smaller particles. Delicate and friable tablets would be damaged by this long lasting stay in the fluidized bed.
U.S. Pat. No. 3,253,944 (Wurster) discloses a process in which the particles to be coated are subjected to a cyclic flow. Instead of the randomness of particle motion characteristic of fluidized beds, a portion of the particles flow upwards, while being sprayed, and the rest of the particles flow downwards. The flow is created by introducing drying and flowing air at different intensity through various parts of the bottom of the drying chamber, for instance by having holes or other perforations distributed in a certain pattern in said bottom. However, it has turned out that the upward flow of particles being sprayed and the downward flow of particles being dried are not easily kept separate and mutual contact between said two particle flows substantially disturbes the process.
A further improvement in coating technology was therefore obtained by introducing a tube or partition located around the perforations where the process air enters and where the spray nozzle is located. Examples of such equipment are described in U.S. Pat. No. 3,241,520 (Wurster et al.). The tube acting as partition solved 2 major problems of the spouting bed: The product layer could be increased because the tube allowed free passage of the coated product and it solved the problem of interference when more spray nozzles were present in the same housing. This equipment turned out to be very suitable for coating relatively small objects, but it was not suitable for coating tablets. This is due to the fact that the free-fall velocity of a tablet is comparatively high, and the process air velocity has to be above this free-fall velocity to transport the tablets pneumatically. However, this high velocity is such that it often damages the tablets, depending on the strength of these.
Another drawback of this equipment is the formation of agglomerates when using sticky coating solutions. Also formation of deposits of coating material on the surfaces of the tube is a common problem, and the utilization of the drying capacity of the process air is inadequate. Also serious upscaling problems are inherent in this design.
The agglomeration problem was essentially solved by a new apparatus described in WO 95/20432 (Aeromatic-Fielder AG) in which the process air was imparted a swirling motion already before reaching the bottom plate of the apparatus, and the process air was introduced just around the upward directed nozzle. Although this apparatus involved substantial improvements and was capable of producing more uniform high-quality coatings than other apparatuses it was less suitable for large tablets than for minor objects.
This is partly due to the fact that the object to be coated has to be in a spinning movement when hit by the spray of atomized coating liquid droplets.
In the apparatus described in the above-mentioned WO 95/20432, the particles to be coated are imparted a suitable spin by the shear flow in the process air. However, this method is not suitable for objects of the size of pharmaceutical tablets.
Therefore, there is a need for a new process and a new apparatus capable of creating the desired fast spin of the object to be coated, also when this object is a relatively large tablet.
Furthermore, the development of tablet pressing machines and other manufacturing equipment has involved a substantial increase of the production capacity thereof and, consequently, there is a need for an increase of also the capacity of coating processes and apparatuses.
Besides, especially in the pharmaceutical industry, there is an increasing need for processes producing a very precise coating. That means that all tablets in a batch, or in a lot being treated continuously, must receive substantially the same predetermined amount of coating material, and, this must form a film or layer of even thickness on all surfaces of each tablet. This is important when a purpose of the coating is to obtain a precise sustained release of drug from the tablet having received the coating or when the coating in itself comprises an active ingredient.
Also for the production of multi-layered coating there is a need of a precise coating as defined above.
It has turned out that the presence of partitions, such as the tubes used in the embodiments of the above-mentioned U.S. Pat. No. 3,241,520 and WO 95/20432, for tablet coating not only involves problems due to the abrasion of the tablets thereon and the formation of sticky deposits, but also because the construction, using partitions outside which a thick layer of objects to be coated is resting, demands a long residence time for the product resulting in a low production capacity and a long lasting mechanical stress on the tablet.